The PSA recurrence rate was 27.2% among men in the soy supplement group versus 29.5% among those in the calcium-derived control group in an interim analysis. The study was discontinued early after similar results were found in an analysis limited to adherent patients.

"I think this study clearly demonstrates men in this particular situation… will not benefit," Maarten C. Bosland, DVSc, PhD, of the University of Illinois at Chicago, and colleagues reported in the Journal of the American Medical Association.

However, the researchers noted that the negative study findings only apply to a specific patient population -- men at above-average risk for PSA recurrence within the first two years after surgery, who receive the specific soy dose tested -- and may not be generalizable to prostate cancer patients at average risk of recurrence.

Prostate cancer is the second most prevalent cancer in men after skin cancer, affecting one in six U.S. men, the American Cancer Society estimates. Researchers have noticed reduced instances of prostate cancer in Asian countries, where soy is included more heavily in the average diet, prompting some to wondered whether it has a protective effect.

Several previous studies examining the effect of soy on prostate cancer in the general population have yielded inconclusive results, but none have looked at the nutrient's effect on biochemical prostate cancer recurrence after radical prostatectomy.

So Bosland and colleagues randomly assigned 177 patients from seven U.S. cancer centers between July 2007 and May 2010 to treatment with a soy protein isolate or a caseinate-based product administered via a beverage powder consumed daily.

Eligible participants had one or more high-risk features including PSA >20 ng/mL, final Gleason score ≥8, positive surgical margins, extracapsular extension, seminal vesicle invasion, or micrometastases in pelvic lymph nodes.

“Soy consumption has been suggested to reduce risk or recurrence of prostate cancer, but this has not been tested in a randomized trial with prostate cancer as the end point,” the researchers remarked.

The researchers used a randomized, double-blind trial on 177 men from seven US cancer centers between July 1997 and May 2010. Patients were only considered at high risk if they had one or more of the following criteria: prostate-specific antigen (PSA) levels of greater than 20 ng/mL, a final Gleason score of at least eight, positive surgical margins, extracapsular extension, seminal vesicle invasion, or micrometastases in pelvic lymph nodes. In addition, the patients could be no longer than four months removed from their prostatectomy.

The participants were divided into a group who consumed a daily serving of 20 grams of soy protein isolate and a placebo group who consumed 20 grams of calcium caseinate. Adherence, tested via self-reports and serial measurement of serum genistein, was over 90%.

The researchers defined recurrence as a serum PSA value of at least 0.07 ng/mL seen in consecutive follow up tests.

The trial was supposed to last two years, but was ended early when it insufficient results were seen at an interim follow-up. At that interim analysis, 22 of 81 (27.2%) evaluable patients in the soy arm had recurrence compared with 23 of 78 (29.5%) in the control group. This difference was not significant. Overall, 28.3% of participants developed recurrence.

No adverse effects related to either diet were reported.

Despite these results, the researchers noted that their findings could not be applied beyond the specific cohort studied.

"The lack of protective activity of soy against prostate cancer recurrence observed in this study was limited to men at above-average risk of recurrence within the first 2 years after surgery and to the soy protein dose tested," they explained.

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